Information storage device capable of oepration in a velocity control mode and a tracking control mode

ABSTRACT

In a disk drive device comprising an optical head having a movable part including a lens for focusing a light spot on a track of an optical disk and a photodetector which receives the light reflected from the track and provides a photoelectric conversion signal, a head actuator moves the movable part in a radial direction of the optical disk when the optical head accesses the tracks of the optical disk, and a motion detection means receives the photoelectric conversion signal and outputs a track-traverse motion signal representing the track-traverse motion of the light spot. An acceleration detecting means detects the acceleration of the head actuator. A target velocity generating means generates a track-traversing target velocity determined by the output of acceleration detecting means and the photoelectric conversion signal. A state-observer means receives the output signals from the acceleration detecting means and the motion detecting means, and outputs an estimated track-traverse velocity of the light spot. A head actuator drive circuit controls the head actuator such that the estimated velocity coincides with the target velocity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 07/582,647,filed Sep. 13, 1990, now U.S. Pat. No. 5,285,431, which in turn is acontinuation-in-part of application Ser. No. 07/492,315, filed Mar. 5,1990, now abandoned which in turn is a continuation of application Ser.No. 07/582,647, filed Sep. 13, 1990, now U.S. Pat. No. 5,285,431, whichin turn is a continuation of application Ser. No. 07/127,391, filed Dec.2, 1987, now abandoned.

FIELD OF THE INVENTION

The present invention relates to an optical disk drive device, and moreparticularly, to an optical disk drive device which controls access of alight spot to an arbitrary track on an optical disk.

The present invention also relates to optical disk devices or similarinformation storage devices which have a head movable with respect to anoptical disk, etc., and can write, read or erase the information.

The present invention also relates to an information storage devicecapable of recording, reproducing, or erasing the information on or froman information storage medium having multiple tracks. More specifically,the invention relates to an information storage device, which even inthe case of failure of a seeking operation or the like can be quicklycontrolled for stable operation.

BACKGROUND OF THE INVENTION

FIG. 1 shows a block diagram of an access control system described inJapanese Patent Application No. 101,439/1985 filed by the assignee ofthe present application on May 15, 1985 for an "Optical Disk DriveDevice".

As illustrated, an optical disk 101 has a plurality of recording trackson the disk. The tracks comprise series of pits disposed at a highdensity and arranged in a circular or spiral form. The disk 101 isfitted onto a spindle and is rotated by a disk-drive motor 102. Thedisk-drive motor 102 is rotated under the control of a disk-motor-drivecontrol system 103.

An optical head 104 forms a light spot on the optical disk 101. Thelight spot is moved in the radial direction of the optical disk 101. Theoptical head 104 comprises a frame 105, a source of light such as asemiconductor laser 106, a collimating lens 107, a polarization beamsplitter 108, a λ/4 plate 109, an optical-path changing mirror 110, anobjective lens 111 which focuses the light beam from the light source106 onto the medium surface of the optical disk 101 and forms a lightspot 115 on the surface, a tracking actuator 112 which provides fine ormicroscopic movements of the objective lens 111 in the radial directionof the optical disk 101 for accurate positioning of the light spot on arecording track of the optical disk, and a split-photodetector 113 whichhas a pair of sensor parts adjacent to each other for detecting thereturn light reflected from the optical disk 101, and produces a pair ofelectrical signals corresponding to the amount of light received at therespective sensor parts.

An addition/subtraction amplifying circuit 114 determines the sum of theoutputs from the split-photodetector 113 to produce a sum signal as aninformation signal (reproduced data signal), and determines thedifference between the outputs of the split photodetector 113 to producea difference signal as a tracking error signal. The tracking errorsignal is supplied to a track-traverse counter 118 and a speed-detectingcircuit 120.

The track-traversing counter 118 receives the output signal from theaddition/subtraction amplifying circuit 114 and detects the number oftracks traversed by the optical head 104. An output of this counter 118is supplied to a target-velocity generation circuit 119.

The target-velocity generation circuit 119 receives the output signal ofthe track-traversing counter 118, and, at the time of access, generatesa target-velocity signal for the light spot 115. The target-velocitysignal is sent to a head-actuator drive control circuit 117.

The head-actuator drive control circuit 117 also receives an outputsignal from a polarity switching circuit 2. On the basis of thesesignals, the head actuator drive control circuit 117 controls the driveof a head actuator 116, such as a linear actuator.

When the head actuator 116 is driven through the head-actuator drivecontrol circuit 117 to move the optical head 104 in the radial directionof the optical disk 101.

A speed detection circuit 120 detects the track-traverse speed (thespeed with which light spot 115 traverses the tracks on the optical disk115). The output of the speed detection circuit 120 is fed to thepolarity switching circuit 2. The speed detection circuit 120, togetherwith the polarity-switching circuit 2, forms a velocity-detectioncircuit 124.

The polarity-switching circuit 2 receives an output signal from anaccess-direction command generation circuit 123. Under the control ofthe output signal of the access-direction command generation circuit123, the polarity-switching circuit 2 changes the polarity of the outputof the speed detection circuit 120. More specifically, the detectedspeed (a scalar value) is converted into a detected velocity (a vectorvalue) which also shows the direction.

FIG. 2 is a transfer-function block diagram of the velocity-controlsystem which represents the diagram of FIG. 1. In this drawing, theresults of the subtraction between the output signal V_(S) * of thevelocity detection circuit 124 and the output signal V_(r) of thetarget-velocity generation circuit 119 is input to a gain compensationcircuit 5. The gain compensation circuit 5 determines the frequency bandof the velocity control system.

The gain compensation circuit 5, as well as a notch filter 122 and ahead-actuator drive circuit 6, are built into the head-actuator drivecontrol circuit 117. The notch filter 122 compensates the mechanicalresonance characteristics G_(L) (S) of a block 6 in the head actuator116.

The head-actuator drive circuit 6 is normally of a current drive type,and also contains a drive current detection circuit.

A block 7 in the head actuator 116 represents a force constant of thehead actuator 116. The block 8 represents transfer characteristics. Itsinput is an acceleration, and its output is a head velocity V_(L) (thevelocity with which the optical head 105 is moved by the head actuator116). M designates the mass of the movable part, G_(L) (S) designatesthe mechanical resonance characteristics of the head actuator, and Srepresents Laplacean. K_(V) represents the sensitivity in the velocitydetection of the target-velocity generation circuit 119 and thevelocity-detection circuit 124, and τ represents the track traverseperiod (period taken for the light spot 115 to traverse a track).

FIG. 3 shows an example of gain characteristic of G_(L) (S) in the block8 in FIG. 2 which is shown to have a large resonance peak at a certainfrequency ω_(L) (usually in the order of kHz). FIG. 4 on the other handillustrates the gain characteristics |G_(N) (S)| of the notch filter 122in FIG. 2. G_(n) (S) is selected so that:

    |G.sub.N (S)|≃|1/G.sub.L (S)|,

when ω_(N) =ω_(L).

FIGS. 5 and 6 show open-loop characteristics of the system of FIG. 2.FIG. 5 shows a case where G_(N) (S)=1/G_(L) (S), while FIG. 6 shows acase where G_(N) (S)≠1/G_(L) (S).

The system operates in the following manner:

First, the disk drive motor 102 is energized through thedisk-drive-motor control circuit 103, and the optical disk 101 shown inFIG. 1 begins to rotate. When the rotation speed reaches a predeterminedsteady value, the tracking actuator 112 is controlled on the basis of atracking error signal obtained by the photodetector 113 andaddition/subtraction amplifying circuit 114. As a result, the light spot115 begins to follow the center of a track on the optical disk 101.

At the time of track access, the number of tracks on the optical disk101, which have been traversed by the light spot 115, is counted by thetrack-traversing counter 118, and, at the same time, in accordance withthe number of tracks to be traversed to reach the target track, thetarget velocity, which is output from the target-velocity generationcircuit 119, and the track-traverse velocity of the light spot 115,which is detected by the velocity-detection circuit 124, are input tothe head-actuator drive control circuit 117, which performs velocitycontrol in which the velocity is reduced to zero as the light spot 115approaches the the target track.

Operation of the velocity control system of FIG. 2 at the time of trackaccess will now be described in detail. A velocity deviation signal Ve,which is the difference between the target-velocity signal V_(r) fromthe target-velocity generation circuit 119 and the detected-velocitysignal V_(s) * from the velocity detector 124, is transmitted to theactuator drive circuit 6 through the gain-compensation circuit 5 and thenotch filter 122. As a result, a certain drive current is applied to thehead actuator 116. Due to this drive current, the head actuator 116begins to move the optical head 104, causing the light spot 115 totraverse the tracks on the optical disk 101.

If the track fluctuation velocity, due for example to eccentricity ofthe optical disk 101, is denoted by Vd, the difference between thevelocity of the head actuator 116 and the track fluctuation velocityV_(o) is detected by the velocity detection circuit 124 as adetected-velocity signal V_(S) *. This detected-velocity signal V_(S) *is fed back in a velocity control system and the control is so made thatthe detected-velocity signal V_(S) * coincides with the target-velocitysignal V_(r).

A loop transfer function (open-loop characteristics) of thisvelocity-control system from the velocity-deviation signal Ve to thedetected-velocity signal v_(S) * can be expressed as follows: ##EQU1##

If it is so designed that the condition G_(N) (S)=1/G_(L) (S) issatisfied, the head actuator does not have the resonance frequency inthe high-frequency zone, as shown in the upper part of FIG. 5. But ifG_(N) (S)≠1/G_(L) (S) because of manufacturing fluctuations betweenindividual devices, the head actuator may have a resonance frequency inthe high-frequency band as shown in FIG. 6. When the peak of thisresonance exceeds 0_(db), the velocity-control system loses itsstability.

Moreover, because of a certain dead time of the zero-order holdcharacteristics of the velocity-detection circuit 124, a long delay inphase is observed in the vicinity of the track-traversing frequency.

Because of the configuration described above, the conventional opticaldisk drive device had the following problems:

(1) The track traverse velocity is detected based on the track traverseperiod. When the traverse is made at a low velocity, a time delay (deadtime) is lengthened, and the velocity control system loses itsstability, and because the cut-off frequency of the velocity-controlsystem must be designed low, the velocity deviation will be increased.

(2) When the light spot 115 traverses drop-out or data address recordingportions, such traverse may erroneously be recognized as traverse oftracks, and, in spite of the slow movement of the light spot, thevelocity detection circuit 124 erroneously operates as if the tracktraverse velocity were high. The result is that the velocity controlsystem is disturbed.

(3) The head actuator 116 typically has a high mechanical resonance atthe frequency of several kHz. In order to eliminate this phenomenon, anotch filter 122 is built in the head actuator drive-control system. If,however, there are a plurality of resonance frequencies, a plurality ofnotch filters need to be provided, and the size of the circuit of thesystem is therefore enlarged. In addition, where there are differencesin the resonance frequency from one device to another, and the resonancefrequencies differ from the frequencies of the notch filters, and thevelocity-control system is not stable.

(4) Because the known system does not detect the direction in whichlight spot 115 traverses the tracks, but simply assumes that the lightspot 115 is traversing the tracks in the direction (toward the inner orouter periphery of the disk) in which the access is to be made anddetermines the required velocity by changing the polarity of the speed.When the speed is low, and the direction in which the track is traversedis reversed due to track fluctuation or disturbances, a positivefeed-back is applied to the system, and the optical disk may behaveerratically.

Another problem of the above-described optical disk drive device isdescribed below:

As the above-described optical disk drive device obtains head positioncontrol information from the optical disk, the absolute position of thehead cannot be reliably detected when operation of the servo-system isdisturbed, e.g., in case of application of a large external impactforce. In that case, as well as in the case of abnormal operation of theservo-system the head may run out of the proper range toward the centeror periphery of the disk and collide with a stopper located at the endof the range of mobility. As a result of this collision, the headreceives a blow and can be broken.

The same problem relates to magnetic disk devices, or any otherequivalent information storage devices.

Another example of prior art is explained below with reference to FIGS.7 and 8.

FIG. 7 illustrates a block diagram which shows a control system of aknown optical disk drive device published in Papers from the GeneralMeeting of the Institute of Electronics and Communications Engineers(IECE) of Japan, 1985, Vol. 7, pp. 7-76 [1170, "Track Access in aTwo-Stage Servo-System", by Hiroshi Inada and Shigeru Shimono]. FIG. 8illustrates waveforms of control signals used in connection with thedevice shown in the block diagram. In these drawings, reference numeral201 designates an optical disk for recording information, or withinformation already recorded on tracks which are arranged in the form ofequally-spaced concentric circles or in the form of a spiral. Referencenumeral 202 designates a light beam by means of which information istransferred to and from the optical disk. A head actuator, e.g., alinear actuator 205 drives a carriage 204 of an optical head 203 andmoves the carriage 204 with respect to the optical disk 201 and acrossthe tracks. A tracking actuator 206 is installed on the carriage 204 andcarries a focusing lens for the formation of a spot of light beam 202 onthe tracks of the optical disk 201. The tracking actuator 206 is movedin the same direction as the linear actuator 205 and can cover only arelatively small, predetermined number of tracks. A split photodetector207 which detects the information signal transmitted by the optical beam202 and converts it into an electrical signal and outputs the electricalsignal. A sensor of this detector consists of two parts. Each such partof the sensor produces on its output an electric signal corresponding tothe quantity of light of the light beam 202 which is incident on thispart.

A subtraction amplifier 211 receives a signal from each sensor part ofthe split photodetector 207, performs subtraction, and thus detectsdeviation of light spot of beam 202 from the center of the track onoptical disk 201. A velocity detection circuit 212 detects, on the basisof an output signal from the subtraction amplifier 211, the tracktraverse velocity (the velocity with which the light spot of beam 202traverses the tracks of optical disk 201 in its movement across thedisk). A pulse generation circuit 213 receives signals from thesubtraction amplifier 211 and generates a pulse each time the light spotof the beam 202 crosses a track on the disk. A track counter 214receives a signal corresponding to the track access number N (the numberof the tracks that must be traversed to reach the target track from theinitial (currently-positioned) track) supplied from outside. The trackcounter 214 receives pulse signals from the pulse generation circuit 213and counts down by "1" each time a pulse is applied to it, and its countvalue is the remaining tracks to be traversed to reach the target track.A reference velocity generation circuit 215 receives from the trackcounter 214 a signal corresponding to the remaining number of tracks,initially determines the reference velocity pattern corresponding to thenumber of the remaining track, memorizes this pattern, and thensequentially produces on its output the reference velocity signalscorresponding to gradual decrease in the number of remaining trackscounted by the counter 214. A velocity error detector 216 receives areference velocity signal from the reference velocity generation circuit215 and a light spot velocity signal from the velocity detection circuit212, and which detects the difference in velocities. An amplifyingcircuit 217 amplifies an output signal of the velocity error detectioncircuit 216 and controls the linear actuator 205. A position controlcommand circuit 218 receives signals from the operational amplifier 211,the velocity control circuit 212, and the track counter 214. When on apredetermined track the velocity of the light spot of beam 202 dropsbelow a predetermined value, the position control command circuit 218produces a position control command on its output. A trackingservo-circuit 219 receives a position control command from the positioncontrol command circuit 218 and thus controls operation of the trackingactuator 206.

The above-described conventional optical disk drive device operates asfollows: Track-access control is comprised of a velocity control modeand a position control mode. In the velocity control mode, the carriage204 is driven by the linear actuator 205, to cause movement of the lightspot in the direction of traverse of the tracks of the optical disk 201.In the position control mode, after the velocity of the light spot ofthe light beam 202 has been reduced below a predetermined velocity atthe predetermined track, the tracking actuator 206 is controlled and thelight spot is stopped at the position where the spot coincides with thecenter of a track on the disk 201 (FIGS. 8A-8C). First, in the velocitycontrol mode, a signal which corresponds to track access number suppliedfrom outside (number N in FIG. 7) is sent to the track counter 214.Because at the very beginning there are no pulses from the pulsegeneration circuit 213, so the number of the remaining tracks is leftunchanged and the generated signal corresponds to this particular numberN. Receiving this signal, the reference velocity generation circuit 215initially determines the reference velocity pattern (FIG. 8A), and thensequentially outputs reference velocity signals in accordance with thenumber of remaining tracks as counted by the track counter 214. Thereference velocity signal and the light spot velocity signal, which isproduced by the velocity detection circuit 212, are input to thevelocity error detection circuit 216 where both signals (i.e., ofdetected and reference velocities) are compared. The difference isamplified by the amplifier 217, and the amplified signal is used tocontrol the velocity of the linear actuator 205. In accordance with thereference velocity pattern, the linear actuator 205 makes accelerationup to a predetermined number of tracks, the velocity is then stabilizeduntil a predetermined number of tracks is reached, when the decelerationis made.

In this way, the light spot of beam 202 moves across the optical disk toreach the target track. When the light spot of beam 202 traverses atrack, the quantity of light reflected from the optical disk 1 willchange. As the sensor of photodetector 207 consists of two parts, thequantity of light reflected onto each sensor part also will vary. Thelight reflected onto the sensor is converted to electrical signals whichcorrespond to the amount of light received by the sensor and which areoutput from the sensor parts. The output signals from the sensor partsare input to the subtraction amplifier 211 performs subtraction toproduce a difference signal as shown in FIG. 8D. In this differencesignal waveform, a zero point of each cycle corresponds to the momentwhen the center of the track on optical disk 1 coincides with the centerof the light spot of beam 202. The velocity detection circuit 212receives the output difference signals from the subtraction amplifier211 and detects on the basis of these signals the track traversevelocity. The pulse generation circuit 213 generates pulses, forexample, at the moment of each cycle when the difference signal waveformof the output from the subtraction amplifier 211 passes through zero.Each such pulse is used as a signal indicating that the light spot ofbeam 202 crossed the track. The pulses are supplied to the track counter214. The position control command circuit 218 receives output signals ofthe subtraction amplifier 211, the velocity detection circuit 212, andthe track counter 214. If at the moment of arrival of the light spot ata position with a predetermined number of tracks to the target track,e.g., one track to the target one, the velocity is below a predeterminedvalue, the position control command circuit 218 will issue an outputcommand which will switch the system to the position control mode.

In the position control mode, the tracking servocircuit 219 receives theoutput signals of the position control command circuit 218 and thesubtraction amplifier 211, and controls the tracking actuator 206referring to the phase of the difference signal waveform from thesubtraction amplifier 211. When the center of the target track of theoptical disk 201 coincides with the center of the light spot of beam202, the tracking actuator stops. Thus, pull-in into the track iscompleted. The light spot of beam 202 follows the target track, andrecording and reproduction of information is conducted.

In the known optical disk drive device of the type described above, thecarriage 204 is driven by the linear actuator 205, and when the targettrack is reached and operation of the linear actuator 205 and trackingactuator 206 is switched from the velocity control mode to the trackingcontrol mode, the detected speed may be disturbed either by defects inoptical disk 201, or by sudden deviation in the actual velocity due forexample to external forces. As a result, the pull-in by the trackingservocircuit 219 may not be achieved. In such a case, the system maybehave erratically, unless an external position or velocity scale isprovided.

SUMMARY OF THE INVENTION

It is an object of the present invention to compensate the dead time ofthe velocity-detection circuit, increase the stability of operation ofthe velocity-control system, widen the operating frequency-band of thesystem, reduce deviation in the velocity, decrease disturbances of thevelocity-control system caused by erroneous operation of thevelocity-detection circuit at the moment of passage over drop-out andaddress data portions of the disk, eliminate the notch filter therebysimplifying the circuit, and suppress the influence of the mechanicalresonance at any frequency.

Another object of the invention is to protect the optical head frombehaving erratically, even when the track traverse direction is reverseddue to disturbances or track fluctuations.

Another object of the present invention is to provide such aninformation storage device, which protects the head from "run-out" underany extraordinary circumstances, protects the head from collision andbreakage, and provides stable operation at the starting period.

Another object of the present invention is to provide such aninformation storage device for recording, reproducing, and erasinginformation on or from the information storage medium, which does notrequire an external scale (which some conventional system employ) andwhich is capable of avoiding erratic behavior in the event of an offtrack (departure from the target track) which may occur during trackingmode due for example to external forces, and which is capable ofreturning the light spot to the area in the vicinity of the target trackand of continuing its operation in the tracking mode.

An optical disk drive device according to the invention comprises:

an optical head which forms a light spot on an optical disk withmultiple tracks, said optical head including an photodetector whichreceives light reflected from said tracks and provides a photoelectricconversion signal, and a movable part including a lens for focusing saidlight spot on said optical disk;

a head actuator which is connected to said optical head and which, whensaid optical head accesses said tracks of said optical disk, moves saidmovable part in a radial direction of said optical disk;

a motion detection means which is connected to said optical head, andwhich receives said photoelectric conversion signal produced by saidphotodetector and produces as output a track-traverse motion signalrepresenting the track-traverse motion of said light spot;

an acceleration detecting means for detecting the acceleration of saidhead actuator and producing an output signal representative thereof;

a target velocity generating means connected to said accelerationdetection means for generating a track-traversing target velocitydetermined by the output of acceleration detecting means and saidphotoelectric conversion signal;

a state-observer means which is connected to said acceleration detectingmeans and said motion detecting means, and which receives as input theoutput signals from said acceleration detecting means and said motiondetecting means, and produces as output an estimated track-traversevelocity of said light spot; and

a head actuator drive circuit connected to said target velocitygeneration means and said state-observer means for controlling said headactuator such that the estimated velocity coincides with said targetvelocity.

According to a further aspect of the invention, there is provided aninformation storage device having a head which can move with respect toan information-storing medium and write, read, or erase thisinformation. The device is provided with a head position detector whichdetects the position of the head between the limits of the userutilizable region of said information storing medium and limits ofmobility of said head, and operates so that when the head exceeds theabove-mentioned limits of the user utilizable region, or receives a stopcommand, the head is moved to a position associated with theabove-mentioned detector.

When for any reason the head runs out of its proper range, it willexceed the limits of the user utilizable region and reach the positionof the head detector. The latter detects the head, moves it to aposition associated with the detector, and thus protects the head fromreaching the limits of its mobility, and hence, from collision with thestopper.

According to a further aspect of the invention, there is provided aninformation storage device which comprises: a head for recording,reproducing, and erasing the information on or from the informationstorage medium, head drive means for driving a movable portion of thehead in a track-traverse direction; tracking control means which allowsthe tip of the head to follow the center of the track; off-trackdetection means; and means for controlling the velocity with which thetip of the head traverses the tracks.

When an off-track (departure of the light spot from the target track)occurs during tracking control, this is is detected, and the trackingcontrol is interrupted, the track traverse velocity is controlled, andwhen the track traverse velocity is reduced below a value at whichpull-in into the tracking is possible, the tracking control is resumed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a block diagram of a known optical disk drive device.

FIG. 2 is a block diagram of the velocity-control system used in theknown optical disk drive device.

FIG. 3 shows high-frequency-band resonance frequency characteristics ofthe mechanical system.

FIG. 4 shows an example of frequency characteristics of a notch filterbuilt in the velocity-control system of FIG. 2.

FIGS. 5 and 6 respectively show examples of the open-loopcharacteristics of the velocity-control system shown in FIG. 2.

FIG. 7 is a diagram of another known device.

FIG. 8 shows waveform explaining operation of the known device of FIG.7.

FIG. 9 is a block diagram of the optical disk drive device of anembodiment of the invention.

FIG. 10 is a block diagram of the velocity-control system incorporatedin the optical disk drive device.

FIG. 11 is a diagram which explains the operation of the state-observerunit in the optical disk drive device.

FIG. 12 is a diagram which explains the operation of thedirection-detection circuit of the optical disk drive device.

FIG. 13 is an example of an open-loop transfer characteristic of thevelocity-control system of FIG. 10.

FIG. 14 is a block diagram of a velocity-control system of the opticaldisk drive system of another embodiment of the invention.

FIG. 15 is a diagram showing a modification of the optical disk drivesystem of FIG. 9.

FIG. 16 is a block diagram of the optical disk drive device of anotherembodiment of the invention.

FIG. 17 is a block diagram of the velocity-control system incorporatedin the optical disk drive device.

FIG. 18 is a block diagram of another velocity-control system which canbe built in the same optical disk drive device.

FIG. 19 a block diagram of another velocity-control system built in theoptical control system.

FIG. 20 is a diagram which explains the operation of thetrack-traversing detection circuit in the optical disk drive device.

FIG. 21 is a diagram which explains the operation of thedirection-detection circuit of the optical disk drive device.

FIG. 22 is an example of an open-loop transfer characteristics of thevelocity-control system of FIG. 17.

FIG. 23 is a block diagram of the device of another embodiment of theinvention.

FIGS. 24 and 25 show electric circuit diagrams of the elements of thedevice.

FIGS. 26 to 28 are time charts which illustrate the operation of thedevice.

FIGS. 29 to 31 are explanatory diagrams which show the area of linearoutput signals.

FIG. 32 is a block diagram illustrating a device of in anotherembodiment of the invention.

FIG. 33 is block diagram of a velocity control system for the device.

FIG. 34 is a diagram which illustrates some details of the elements ofthe system.

FIG. 35 is a diagram showing details of the track traverse velocitydetection means used in the device of the above embodiment.

FIGS. 36 to 38 show waveforms of signals used for explanation ofoperation of the device.

FIG. 39 is a block diagram of a modification of the device of the aboveembodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 9 shows a block diagram of a system of an embodiment of the presentinvention. In this drawing, reference numerals 10 to 120 designateidentical or corresponding elements in the conventional system shown inFIG. 1, and their description is omitted in order to avoid duplicationof explanation, and the following explanation is concentrated mainly onelements which are different from those shown in FIG. 1.

A direction-detection circuit 1 receives the difference signal and thesum signal from the addition/subtraction amplifying circuit 114. On thebasis of these signals, the direction detection circuit 1 detects thetrack traverse direction (direction with which the light spot 115traverses the tracks). Responsive to an output signal from thedirection-detection circuit 1, a polarity switching circuit 2 determinesor switches the polarity of an output of the speed-detection circuit120. A velocity-detection circuit 20 is composed of thedirection-detection circuit 1, the speed-detection circuit 120, and thepolarity switching circuit 2.

A state-observer unit 3 receives the drive current signal detected bythe drive current detection circuit 121, which detects the drive currentof a head actuator 116, and a velocity-detection signal which has apolarity appended at the polarity switching circuit 2. On the basis ofthe above signals, the state-observer unit 3 presumes or estimates thevelocity which is closer to the true value. An output signal from thestate-observer unit 3 is sent to the head-actuator drive control circuit117.

When a light spot 115 moves along the center of the track, acontrol-mode detection circuit 4 generates a command which resets anintegrator 12 in the state-observer circuit 3.

FIG. 10 is a transfer-function block diagram showing thevelocity-control system of FIG. 9. In this drawing, the state-observerunit 3, the control-mode detection circuit 4, and the velocity-detectioncircuit 20 correspond to those of FIG. 9. In addition, again-compensation circuit 5, a force constant 7 of the head actuator,and block 8 are the same as those in FIG. 8.

The state-observer unit 3 is comprised of gain elements 9, 10 and 13, afeedback gain element 11, an integrator 12, a subtractor 14 and an adder15.

The gain element 9 has a gain K_(F) equivalent to the force constant 7of the head actuator 116, receives the drive current which is detectedin the head actuator drive circuit 6, and outputs an estimated value ofthe drive force. The gain element 10 has a gain 1/M which is thereciprocal of the mass of the movable parts of the head actuator 116 andthe optical head 104 which are movable at the time of access. The outputof the gain element is an estimated value of acceleration of the head.The subtractor 14 determines the difference between the detectedvelocity V_(S) * from the velocity detection circuit 20 and the outputof the gain element 13, which is the estimated track traverse velocityV_(S), as will be apparent from the subsequent description. The feedbackgain element 11 receives the output of the subtractor 14. The adder 15determines the sum of the estimated acceleration from the gain element10 and the output of the feedback gain element 11. The integrator 12integrates the sum as output from the adder 15. The integrator 12 isreset by an output from the control-mode detection circuit 4. The gainelement 13 simulates the velocity-detection circuit 20 and its output isthe estimated velocity signal V_(S).

Of the above described elements, the gain elements 9 and 10 incombination form a means for simulating the blocks 7 and 8, i.e.,nominal characteristics of the head actuator 116. The output of the gainelement 10 is a simulation of the acceleration of the optical head. Thefeedback gain element 11, the integrator 12, the gain element 13, thesubtractor 14 and the adder 15 in combination form a means for combiningthe output of the simulating means (9 and 10), and the detected velocityV*. Simply stated, its output, which is the estimated velocity, is givenas the sum of the detected-velocity V_(S) * (which is updated each timetraverse of a track is detected) and an estimated head velocity (asobtained by the simulation by the use of the gain elements 9 and 10 andthe subsequent integration by the integrator 12). For low frequencycomponents, the detected-velocity V_(S) * is dominant, i.e., V_(S)≃V_(S) *. For high frequency components, the estimated value of the headvelocity is dominant, i.e., V_(S) ≃V_(L). This will be later describedin further detail.

FIG. 11 shows waveforms of signals appearing at various parts of thevelocity-control system. Reference numeral 14 designates atrack-traversing sensor signal obtained from the output ofaddition/subtraction amplifying circuit 114 of FIG. 9. Reference numeral15 designates an output signal (detected velocity signal) V_(S) * of thevelocity-detection circuit 20. Reference numeral 16 designates anestimated velocity signal V_(S) at the output of state-observer unit 3.Reference numeral 17 designates an estimated velocity signal V_(S) whichappears on the output of the state-observer unit 3, in the case when thegain of feedback gain element of FIG. 10 is equal to zero (L=0).

FIG. 12 shows the relationship between track grooves and the detectedsignals. FIG. 12(a) is a cross-sectional view of an optical disk. Inthis drawing, reference numeral 18 designates a groove portion and 19designates portion between the grooves. FIG. 12(b) shows a differencesignal (tracking error signal) from the addition/subtraction amplifyingcircuit 114 of FIG. 9. FIG. 12(c) illustrates a sum signal (informationsignal) from the addition/subtraction amplifying circuit 114. FIGS.12(d) and (e) are respective comparator signals (which are obtained bydigitizing the analog signals shown in FIGS. 12(b) and 12(c) into binarysignals.

FIG. 13 shows respective open-loop transfer characteristics of thecontrol system shown in FIG. 10, representing the frequencycharacteristics of the gain and phase.

In the optical disk drive device as described above, light which isemitted from a light source 106 is collimated by the collimator lens107, passes through a polarization beam splitter 108 so that the lightemitted therefrom is linearly polarized, passes through a λ/4 plate, isreflected by a mirror 110, and is then converged by an objective lens111 into a light spot 115 on the surface of the optical disk 101rotating at a steady speed.

The light reflected from the optical disk 101 passes through theobjective lens 111, is reflected at the polarization beam splitter 108,and is sent to a split-photodetector 113.

The light received by the split-photodetector unit 113 isphoto-electrically converted by the split-photodetector 113 intoelectrical signals. In the addition/subtraction amplifying circuit 114,the electrical signals from the split-photodetector 113 are added andsubtracted to form the sum signal (information signal) and thedifference signal (tracking-error signal).

At the time of access, the sum signal and tracking error signal are sentto the inputs of the track-traversing counter 118, the speed-detectioncircuit 120, and the direction-detection circuit 1.

As the direction-detection circuit 1 detects the track-traversedirection, i.e., whether the light spot 115 is moved outward (toward theperiphery of the disk) or inward (toward the center of the disk), thepolarity is determined or switched in the polarity switching circuit 2.After the switching of the polarity, the signal is supplied as adetected velocity signal to the input of the state-observer unit 3.Simultaneously, the drive current signal of the head actuator 116 whichis detected by the drive current detection circuit 121 is also input tothe state-observer unit 3.

Furthermore, at this moment, an output signal of the control-modedetection circuit 4 clears the reset of the integrator 12 in thestate-observer unit 3, so that the state-observer unit 3 is activated.

Meanwhile, an output signal of track-traversing counter 118 istransmitted to the target-velocity generation circuit 119 and the outputof the target-velocity-speed generation circuit 119 forms a signal whichcorresponds to the target velocity related to the number of theremaining tracks (tracks to be traversed to reach the target track).

The head-actuator drive control circuit 117 receives the output signalsfrom the target-velocity generation circuit 119, the state-observer unit3, and the drive current detection circuit 121. On the basis of thesesignals, the head-actuator drive control circuit 117 controls theoperation of the head actuator 116, and hence, the track traversevelocity.

Operation of the state-observer unit 3 will now be described withreference to FIGS. 10 and 11. An output signal of the gain-compensationcircuit 5, which determines the frequency-band, i.e., the operatingrange of the velocity-control system, comprises the drive command signalof the head actuator 116. This signal is converted into a drive currentin the head-actuator drive circuit 6, which produces a drive force bymultiplication with a force constant K_(F) [N/A] in the head actuator116 into force constant. The drive force in turn produces anacceleration by multiplication with a factor 1/M which is the reciprocalof the mass. The acceleration is integrated and affected, in thehigh-frequency band, by the resonance characteristics G_(L) (S) of thehead actuator 116. As a result, the head actuator 116 is moved at a headvelocity V_(L).

Due to eccentricity of the optical disk 101, for example, the tracks mayfluctuate with respect to a stationary structure, such as a frame of theoptical disk drive device, not illustrated as such. In such a case, thetrack traverse velocity is then equal to the difference between the headvelocity V_(L) and the track fluctuation velocity Vd (the velocity withwhich the track fluctuates). This velocity is detected in thevelocity-detection circuit 20, and is converted into an electricalsignal by multiplication with a gain which comprises avelocity-detection sensitivity K_(v) [Vm/s].

The detected-velocity signal 15, which is obtained by detection from thetracking periods of the track-traversing sensor signal 14, whichcomprises an output of the addition/subtraction amplifying circuit 114(FIG. 11), is not obtained until track traverse is first detected, andis thereafter obtained as an average velocity of traverse over thepreceding half-track. During deceleration, the detected-velocity signal15 is in the form of steps as illustrated. It thus has a zero-order holdcharacteristics shown in the speed detection circuit 120 (FIG. 6). Asthe velocity is lowered, the track traverse period τ is longer.

The state-observer unit 3 comprises an electronic circuit whichsimulates the nominal transfer characteristics of the head actuator 116and the velocity-detection circuit 20, i.e., transfer characteristicsdisregarding the high-frequency band resonance characteristic G_(L) (S)of the head actuator 116 and the zero-order hold characteristic(1-e^(-S)τ)/Sτ of the velocity-detection circuit 20.

In the state-observer unit 3, the drive current signal I_(L) of the headactuator 116, which is detected in the drive current detection circuit6, is converted through the gain elements 9 and 10 into the accelerationinformation. The adder 15 adds the acceleration information from thegain element 10 to the output of the feedback gain element 11, and thesum is passed through the integrator 12 and then the gain element 13with a gain K_(V). The output of the gain element 13 represents theestimated velocity signal V_(S).

The subtractor 14 determines the difference between the estimatedvelocity V_(S) and the detected-velocity V_(S) *. This difference ismultiplied with L/K_(V) in the feedback gain element 11, and is added tothe output of the gain element 10 at the adder 15. The sum is input tothe integrator 12, as described before. In this way, the differencebetween the estimated velocity signal V_(S) and the detected velocitysignal V_(S) * is multiplied with a gain L/K_(V) and added to theacceleration signal, and the sum is input to the integrator 12, thus theestimated velocity V_(S) and the detected-velocity V_(S) * convergetoward each other (i.e., approaches each other).

The transfer function from the two input signals to the state-observerunit 3, i.e., the drive current signal I_(L) and the detected-velocitysignal V_(S) * to the estimated velocity V_(S) on the output of thestate-observer unit 3, can be expressed as follows:

    V.sub.S =1/(S+L)·K.sub.F K.sub.V /M·I.sub.L +1/(S+L)·V.sub.S *                               (2)

More specifically, the transfer function from the drive current signalI_(L) and to the estimated-velocity signal V_(S), and the transferfunction from the detected-velocity signal V_(S) * to theestimated-velocity signal V_(S), are both of a first-order delay, andtheir time constant is equal to 1/L, so the state-observer unit 3 isstable as long as L>0. The value L is a parameter which determines therate of convergence of the estimated velocity signal V_(S). For example,if converging is to be achieved with the time constant equal to 1 ms, Lis selected equal to 10³.

The significance of the formula (2) will now be considered for each ofthe respective frequency-bands. First of all, because K_(F) I_(L) /Mcorresponds to the acceleration of the optical head 104, the followingrelation holds (if we neglect the terms one or more orders smaller) whenthe track fluctuation velocity Vd is sufficiently lower than thevelocity V_(L) of the optical head 104:

    |V.sub.S *|≃|1/S.K.sub.F K.sub.V /M.I.sub.L |                                     (3)

Substituting S=jω, the following relations hold. (i) When ω<<L (|S|<<L):

    V.sub.S ≃1/L.K.sub.F K.sub.V /M.I.sub.L +V.sub.S *(4)

If L≃1000, from the formula (3) one can obtain the following:

    |V.sub.S *|≃|1/S.K.sub.F K.sub.V /M.I.sub.L |>|1/L.K.sub.F K.sub.V /M.I.sub.L |(5)

Therefore

    V.sub.S ≃V.sub.S *                           (6)

(ii) When ω>>L (|S|>>L), the following relations holds.

    V.sub.S ≃1/S.K.sub.F K.sub.V /M.I.sub.L +L/S.V.sub.S *(7)

From formula (3). the following is derived:

    |L/S.V.sub.S *|<<|V.sub.S *|≃|1/S.K.sub.F K.sub.V /M.I.sub.L |                                                (8)

Therefore,

    V.sub.S ≃1/S.K.sub.F K.sub.V /M.I.sub.L      (9)

is obtained.

It is seen from the formulae (6) and (9), that the estimatedtrack-traverse velocity signal V_(S) is equal to the the detectedvelocity signal V_(S) * in the low-frequency band, while in thehigh-frequency band, it is equal to an integral of the head-actuatordrive current I_(L). The break point (the boundary between the region inwhich the estimated track-traverse velocity signal V_(S) is equal (orabout equal) to the detected velocity signal V_(S) * and the region inwhich the estimated velocity signal V_(S) is equal (or about equal) toan integral of the head-actuator drive current I_(L)) is a frequency L[rad/sec] which coincides with the frequency band of the state-observerunit 3. If, for example, L=∞, the formula (2) will be transformed intothe following:

    V.sub.S =V.sub.S *                                         (10)

The velocity-control system is then identical to a conventional onewhich is shown in FIG. 2 and does not have a state-observer unit. Thetime response of such a system, V_(S), coincides with the output signal15 of FIG. 13, so it is impossible to compensate for the dead time whichoccurs in the velocity-detection circuit 20. If, on the contrary, L isequal to 0, then the formula (2) will be transformed into the followingexpression:

    V.sub.S =1/S.K.sub.F K.sub.V /M.I.sub.L                    (11)

Thus, in this case, the time response of the estimated velocity signalV_(S) coincides with the estimated velocity signal 17 in FIG. 11, and isnot affected by the dead time of the velocity detection circuit 120, andthe the high-frequency band resonance characteristics of head actuator116. However, even a slightest offset superimposed on the drive currentI_(L) will increase the error in the estimated velocity V_(S).

Moreover, the estimated velocity signal V_(S) does not contain the trackfluctuation velocity Vd at all. So, the estimated velocity V_(S) is anestimated value for the head velocity V_(L), rather than the tracktraverse velocity. When the speed of movement of the head is low, andthe head velocity V_(L) is so low that the track fluctuation velocity Vdcannot be ignored, the error in the estimated velocity signal V_(S)(estimated value) of the track traverse velocity is large.

Thus, by setting the gain L of the feedback gain element 11 to besufficiently lower than the high-band resonance frequency of the headactuator 116, and the track traverse frequency (the frequency with whichthe light spot 115 traverses the tracks), and sufficiently higher thanthe track fluctuation fundamental frequency, the time response of theestimated velocity signal V_(S) can be made as shown by the broken line16 in FIG. 11, which is intermediate between the output signals 15 and17. Thus, the dead time of the velocity detection circuit 120 can becompensated for to a certain extent.

Because on the basis of formula (2), the transfer characteristic fromthe detected-velocity signal V_(S) * to the estimated velocity signalV_(S) can be represented by the characteristic of a first-order low-passfilter represented by 1/(1+S/L), the estimated velocity signal V_(S)will not be disturbed substantially, even when the output signal (i.e.,the detected-velocity signal) V_(S) * is disturbed, e.g., under theeffect of recording pits or drop-outs on the optical disk 101.

In addition, by the arrangement in which, during tracking of a track bythe light spot 115, the integrator 12 is reset by a command from thecontrol-mode detection circuit 4, and this reset is clearedsimultaneously with the switching to the velocity-control mode, sincethe track-traverse velocity during the tracking is certainly equal tozero, the initial value of the estimated-velocity output signal from thestate-observer unit 3 immediately after the switching to thevelocity-control mode will not include any error. Moreover, even if anerror is present, it will be converged to zero with the time constant1/L.

The open-loop transfer function of the velocity-control system of FIG.10 can be calculated as follows: ##EQU2##

If ω_(L) designates the frequency at which the high-band resonancecharacteristic G_(L) (S) of the head actuator 116 has a peak, then bysetting the value of L so that L<<ω_(L) is satisfied, the followingrelationship holds:

    |L/S.G.sub.L (S)|.sub.S=jωL =L/ω.sub.L |G.sub.L (jω.sub.L)|              (13)

Therefore, the influence which is exerted by the value of the high-bandresonance peaks of the head actuator 116 on the open-loop characteristicof formula (12) will be suppressed to L/ω_(L) times (L/ω_(L) <<1), andthe high-band resonance peaks of the gain characteristics are as shownin FIG. 13, and are smaller than those shown in FIG. 6.

It is also different from the frequency characteristic of a notch filter122 shown in FIG. 3 in that the high-band resonance frequency ω_(L) willnot be required to be a specific value. That is, if the frequencysatisfies condition ω_(L) >>L, the suppression effect will be obtainedat an arbitrary frequency, and even there are a plurality of peaks theyare suppressed uniformly. Similar to mechanical resonance characteristicG_(L) (S), the phase delay and the gain reduction due to the zero-orderhold characteristic of the velocity-detection circuit 20 in the formula(12), are also alleviated, and the phase of the open-loopcharacteristics extends to the high-frequency band, and the stability ofthe system is improved.

When the light spot 115 traverses the grooves of the tracks as shown inFIG. 12(a), the difference signal (FIG. 12(b)) and the sum signal (FIG.12(c)) from the addition/subtraction amplifier 114 are 90° shiftedrelative to each other. Utilizing this fact, it is possible to determinethe direction of movement of the light spot, i.e., whether it is movingfrom the left to the right in FIG. 12(a), or from the right to the leftin FIG. 12(a). That is, if the level of the signal in FIG. 12(e) is highwhen the signal in FIG. 12(d) rises, or if the level of the signal inFIG. 12(e) is low when the signal in FIG. 12(d) falls, the light spot ismoving from the left to the right. If, on the contrary, the level of thesignal in FIG. 12(e) is low when the signal in FIG. 12(d) rises, or ifthe level of the signal in FIG. 12(e) is high when the signal in FIG.12(d) falls, the light spot is moving from the right to the left.

If, in the manner described above, the track traverse direction (thedirection in which the light spot 115 traverses the tracks) is detectedby the direction-detection circuit 1, and the polarity of the outputsignal of the speed-detection circuit 120 is switched depending on thetrack-traversing direction, positive feedback is voided and theoperation of the velocity-control system will be stable.

Another embodiment of the invention is shown in FIG. 14 which is a blockdiagram similar to FIG. 10. In this system, the state-observer unit 3 isslightly modified from the state-observer unit 3 in FIG. 10. Thetransfer function from the detected velocity signal V_(S) * and thedrive current signal I_(L) to the estimated velocity signal V_(S), aswell as the open-loop transfer function, are also as represented byformulae (2) and (12).

Moreover, although the system shown in FIG. 9 has a polarity switchingcircuit 2 installed directly after the speed-detection circuit 120, whatis essential is that the polarity be switched so that the control systemhas a negative feedback, so that the polarity switching circuit 2 mayalternatively be provided directly after the drive-current detectioncircuit 121 or after the state-observer unit 3.

Furthermore, in the embodiment described above, the track traverse speedand the track traverse direction are detected on the basis of thesignals obtained by determining the sum and the difference on thesplit-photodetector outputs. But they may obtained in other ways. Forexample, when a sample servo system using an optical disk without trackgrooves is employed, the track traverse speed and the track traversedirection may be detected on the basis of an output of the trackingsignal (tracking error signal) detection means or an output of a tracktraverse number detection means and an output of a means for detecting asignal corresponding to the reflected-light total-amount signal.

Furthermore, they may be detected from the address information or thelike of the optical disk.

In the illustrated embodiments, the polarity of the output signal fromspeed-detection circuit 120 was switched or determined on the basis ofthe direction detected in the direction-detection circuit. If, however,the track traverse direction is not reverted during velocity control, orif the time for which the track traverse direction is opposite is short,the estimated velocity signal V_(S) can be determined from the drivecurrent signal I_(L) alone, by means of formula (9). Therefore, thevelocity-control system will never have a positive feedback.Accordingly, as shown in FIG. 15, the polarity of the output signal fromthe speed-detection circuit 120 may be switched at a polarity-switchingcircuit 2 on the basis of the access-direction command 151 supplied froman access-direction command generator 150, before being supplied to thestate-observer unit 3.

In the embodiment of FIG. 10, the head actuator was a linear actuator.The head actuator may alternatively be a rotary-type actuator. In thiscase, the mass M of the movable parts described with reference to FIGS.10, 11 and 12 should be replaced by inertia moment J of movable parts.In other words, the head actuator may have any suitable form. The headactuator need not necessarily drive the entire head, and can be used fordriving part only of the head. What is essential is that it is capableof moving the light spot over a large distance in the radiationdirection of the disk. It may be of such a construction that can serveboth as the tracking actuator and the head actuator.

In the embodiment described, the head actuator drive current is input tothe state-observer unit. However, what is essential is that anacceleration of the optical head or a parameter related to theacceleration be input and used for the determination of the estimatedtrack traverse velocity.

As has been shown above, according to the configurations describedabove, the velocity-control system of the optical drive device isprovided with a state-observer unit which operates at the time oftrack-access. This state-observer unit receives an output signal fromthe velocity-detection circuit and the head-actuator drive currentsignal. On the basis of these signals, the state-observer unit estimatesthe track traverse velocity, which is then used for the velocitycontrol. Accordingly, the stability of the velocity control system isimproved, and the velocity control is enabled during access over a shortdistance as well as access over a long distance. The access time issubstantially reduced, and the performance of the velocity controlsystem does not depend on the fluctuation of the mechanical resonancefrequency of the optical head, or the number of the resonancefrequencies. Assembly of the head actuator and the optical head istherefore facilitated.

FIG. 16 shows a block diagram of a system in accordance with anotherembodiment of the present invention. In this drawing, reference numeralsidentical to those in FIG. 1 designate identical or correspondingelements, and their description is omitted, and the followingexplanation is concentrated mainly on elements which are different fromthose shown in FIG. 1.

A track-traversing detection circuit 20 receives the difference and sumsignals from the addition/subtraction amplifying circuit 114. Thesesignals are used to detect track traverse. A direction-detection circuit1 receives the output from the track-traverse detection circuit 20 fordetecting teh track traverse direction. A track-traversing distancedetection circuit 2A receives signals from the direction-detectioncircuit 1 and, on the basis of these signals. counts up or counts downthe output signal from the track traverse detection circuit 20A.

A state-observer unit 3 receives the drive current signal detected bythe drive current detection circuit 121, which detects the drive currentof the head actuator 116, and a track-traversing distance detectionsignal from the track-traversing distance detection circuit 2A. On thebasis of the above signals, the state-observer unit 3 estimates thetrack traverse velocity which is closer to the true value. An outputsignal from the state-observer unit 3 is sent to a head-actuator drivecontrol circuit 117.

A control-mode detection circuit 4 is a circuit, which when the lightspot 115 is following the center of a track, outputs a command forresetting an integrator which is built in the state-observer unit 3.

FIG. 17 shows a transfer-function block diagram of the velocity-controlsystem of FIG. 16. In FIG. 17, the state-observer unit 3, thecontrol-mode detection circuit 4, and the track-traversing distancedetection circuit 2A correspond to the elements designated with the samereference numerals in FIG. 16. In addition, a gain-compensation circuit5, a head-actuator drive circuit 6, a force-constant block 7 of the headactuator, and mechanical-resonance-characteristic block 8 are the sameas those in FIG. 2.

The state-observer unit 3 is comprised of gain elements 9, 10 and 21,feedback gain elements 11 and 12, a first-order delay element 13, asubtractor 14A and an adder 15A.

The gain element 9 has a gain K_(F) equivalent to the force constant(block 7) of the head actuator 116, receives the drive current signaldetected in the head actuator drive circuit 6, and outputs an estimatedvalue of the drive force. The gain element 10 has a gain 1/M which isthe reciprocal of the mass of the movable part of the head actuator 116,the optical head 104, and the like movable at the time of access. Thefeedback gain elements 11 and 12 receive a track-traversing distancedetection signal X_(S) * from the track-traversing distance detectioncircuit 2A.

The subtractor 14A subtracts the output of the feedback gain element 12from the output of the gain element 10. The output of the subtractor 14Ais an estimated value which is closer to the true acceleration and isinput to the first-order delay element 13. The first-order-delay element13 can be reset by an output signal from the control-mode detectioncircuit 4.

The gain element 21 simulates a track-traversing distance detectioncircuit 2A. The adder 15A adds the output signal from the feedback gainelement 11 and the output signal from the gain element 21, and the sumis obtained at an adder 15A appears on the output of the state-observerunit 3 to serves as an estimated velocity signal V_(S).

FIG. 18 shows another embodiment of a state-observer unit 3, which is amodification equivalent to the state-observer unit 3 in FIG. 17. In FIG.18, reference numerals identical to those in FIG. 17 designate identicalor corresponding elements. In this embodiment, the output from the gainelement 21 and the output of an arithmetic element 21 are added at anadder 16A, and the sum, forming the estimated-velocity signal V_(S), isproduced on the output of the state-observer unit 3. Similar to the casewith first-order delay element 13, a predetermined part of thearithmetic element 22 can be reset by means of an output signal from thecontrol-mode detection circuit 4.

FIG. 19 is block diagram showing transfer functions in a furtherembodiment of the velocity-control system of FIG. 16, which is expresseddifferently from that shown in FIG. 17. Particularly, the state-observerunit 3 is in a different form from those in FIGS. 17 and 18. In FIG. 19,reference numerals identical to those in FIG. 17 designate identical orcorresponding elements. Reference numeral 23 designates a feedback gainelement 23, and reference numerals 24 and 25 designate integrators whichare reset by the control-mode-detection circuit 4.

FIG. 20 shows waveforms which appear at various parts during velocitycontrol. Reference numeral 14 designates a track-traversing sensorsignal obtained at the output of addition/subtraction amplifying circuit114 of FIG. 16. Reference numeral 15 designates an output signal(track-traversing distance detection signal) X_(S) * of thetrack-traversing distance detection circuit 2A. Reference numeral 16designates a true track-traversing distance, which is not detected inreality.

FIG. 21 shows a relationship between the track grooves and the detectedsignals. FIG. 21(a) is a cross-sectional view of an optical disk. Inthis drawing, reference numeral 18 designates a groove portion and 19designates a portion between the grooves. FIG. 21(b) shows a differencesignal (tracking error signal) from the addition/subtraction amplifyingcircuit 114 of FIG. 16. FIG. 21(c) illustrates the sum signal from theaddition/subtraction amplifying circuit 114, and FIGS. 21(d) and (e) arerespective comparator signals for those shown in FIGS. 14(b) and 14(c).

FIG. 22 shows the open-loop transfer characteristics of the velocitycontrol system shown in FIG. 17, showing the frequency characteristicsof the gain and phase.

The system described above operates in the following manner:

A light which is emitted from the light source 106 is collimated by acollimator lens 107, passes through a polarization beam splitter 108.The output of the polarization beam splitter 108 is a linearly polarizedlight and is passed through a λ/4 plate, is reflected by a mirror 110,and is then focused by an objective lens 111 into a light spot on thesurface of an optical disk 101 which is rotating steadily.

The light reflected from the optical disk 101 passes through theobjective lens 111 to the polarization beam splitter 108, where thelight is reflected, and incident onto a split-photodetector 113.

The light received by the split-photodetector 113 is photoelectricallyconverted into electrical signals. The electrical signals are thencombined (added and subtracted) at the addition/subtraction amplifyingcircuit 114, into the sum signal and the tracking error signal.

At the time of tracking, the sum signal and the tracking error signalare passed through the track traverse detection circuit 20A, and thedirection detection circuit 1, to the track-traversing distancedetection circuit 2A and the track traversing counter 118.

The pulsative track traverse detection signals output from the tracktraverse detection circuit 20 are accumulated positively (added) ornegatively (subtracted) at the track-traversing distance detectioncircuit 2A, depending on whether the track traverse direction is outward(toward the outer periphery) or inward (toward the axis), the tracktraverse direction being detected by the detector 1. The result of theaccumulation is input to the state-observer unit 3.

For example, during outward access when the light spot 115 is movedoutward, the pulsative track traverse detection signals are counted upby a suitable counter. During inward access when the light spot 115 ismoved inward, the pulsative track traverse detection signals are counteddown. In this way, the desired total track-traversing distance (asrepresented by the total number of tracks traversed) is detected.

At the same time, the drive current signal of the head actuator 116 asdetected by the drive current detection circuit 121 is also input to thestate-observer unit 3.

The resetting of the integrator in the state-observer unit 3 is thencleared by the output of the control mode detection circuit 4, so thatthe state-observer unit 3 can operate.

The output of the track-traversing counter 118 is transmitted to thetarget velocity generation circuit 119 from which a target velocitycorresponding to the remaining number of tracks is output.

In accordance with the output from the target-velocity generationcircuit 119, the output from the state-observer unit 3, and the outputfrom the drive current detection circuit 121, the head-actuator drivecontrol circuit 117 controls the operation of the head actuator 116, andhence, the track traverse velocity.

Operations of the state-observer unit 3 will now be described withreference to FIGS. 17, 18 and 19. An output signal of thegain-compensation circuit 5, which determines the frequency-band, i.e.,the operating range of the velocity-control system, corresponds to adrive command signal for the head actuator 116. This signal is convertedinto a drive current in the head-actuator drive circuit 6, which ismultiplied with the force constant K_(F) [N/A] in the head actuator 116into the drive force, which is multiplied with 1/M, which is thereciprocal of the mass, into acceleration. The acceleration isintegrated into velocity, and the velocity, in turn, is integrated intoa distance.

When the above-mentioned frequency-band of the velocity control systemis in the high-frequency zone, there is also an influence of the highfrequency-band resonance characteristic G_(L) (S) of the head actuator116. Thus, the head actuator 116 will move for a certain distance X_(L).

When there is a track fluctuation X_(d) due for example to theeccentricity of the optical disk 101, the difference between X_(L) andX_(d) will correspond to the track-traversing distance. This distance isdetected by the track-traversing detection circuit 2A and is converted,by being multiplied with gain K_(X) [V/m], which is the sensitivity,into an electrical signal.

As shown in FIG. 20, the track-traversing distance signal 15, which isdetected on the basis of the the tracking period of the track-traversingsensor signal 14, output from the addition/subtraction amplifyingcircuit 114, is obtained when a track traverse is detected, as anaverage traverse velocity over immediately preceding half a track.During deceleration, the track traverse velocity signal is thereforestepwise, and has a zero-order hold characteristics, as shown at theblock 2A in FIG. 17. As the track traverse velocity is lowered, thetrack traverse period becomes longer. The value of τ is smaller duringhigh velocity operation, and is larger during low velocity operation.

Basically, the state-observer unit 3 comprises electronic circuits whichsimulate the transfer characteristics of the head actuator 116 and thetrack traversing distance detection circuit 2A, excepting the highfrequency-band resonance characteristics G_(L) (S) of the head actuator116 and the zero-order hold characteristic (1-e^(-S)τ)/Sτ of thetrack-traverse-distance detection circuit 2A.

In the state-observer unit 3 shown in FIG. 17, the drive current signalI_(L) of the head actuator 116, which has been detected in thedrive-current detection circuit 6, is converted through the gainelements 9 and 10 into acceleration information.

The subtractor 14A determines the difference between the accelerationinformation and the output of the feedback gain element 12 whichreceives the detected track-traverse distance X_(S) *, and thedifference is then passed through the first-order delay element 13 andthe gain element 21, into a velocity signal. At the adder 15A, thevelocity signal is added to the output of the gain element 11 which alsoreceives the detected track-traverse distance X_(S) *. The resultant sumsignal is the estimated velocity V_(S).

The transfer function from the two inputs to the state-observer unit 3,i.e., the drive current I_(L) of the head actuator, and thetrack-traversing distance X_(S) *, to the output of the state-observerunit 3, i.e., the estimated velocity V_(S) can be expressed as follows:##EQU3##

More specifically, because the time constant of the transfercharacteristics from the drive current signal I_(L) to estimatedvelocity signal V_(S), as well as from the track-traversing distancesignal X_(S) * to the estimated velocity signal V_(S) is equal to 1/L,provided that L>0, the state-observer unit 3 is stable. Value L isparameter which determines the rate of convergence of the estimatedvelocity signal V_(S). For example, if the convergence is to beperformed with the time constant equal to 1 ms, L is selected equal to10³.

Considered below is the significance of the formula (22) for therespective frequency-bands. First of all, because K_(F) I_(L) /Mcorresponds to the acceleration of the optical head 104, if the termsone or more orders smaller may be neglected, the following relation (23)holds provided that the track fluctuation value X_(d) is sufficientlysmaller than the movement distance X_(L) of the optical head 104.##EQU4##

Substituting S=Jω

(I) If ω<<L (|S|<<L), ##EQU5##

If L≃1000, from formula (23) one can obtain the following: ##EQU6##

Therefore

    V.sub.S ≃SX.sub.s *                          (26)

(II) If ω>>L (|S|>>L), ##EQU7##

From formula (23), the following is derived: ##EQU8## Therefore,##EQU9## is obtained.

It is seen from formulae (26) and (29), that the estimated velocitysignal V_(S) is equal to the differential value SX_(S) * of thetrack-traversing distance detection signal X_(S) * in the lowfrequency-band, and is equal to the integral of the head-actuator drivecurrent I_(L) in the high frequency-band. Thus, the frequency formingthe boundary between the above-mentioned high and low frequency-bands isL [rad/sec] which coincides with the frequency band of thestate-observer unit 3. If, for example, if is it assumed that L=∞,formula (22) will be transformed into the following:

    V.sub.S =SX.sub.S *                                        (30)

Because in this case, the drive current I_(L) is not used for thedetermination of the estimated velocity V_(S), and the estimatedvelocity signal V_(S) is given by the differential of thetrack-traversing distance signal X_(S) *, the dead time developed intrack-traversing distance detection circuit 2A could not be compensated.If, on the contrary, L is assumed equal to 0, then formula (22) will betransformed into the following expression: ##EQU10##

Because in this case, the track-traversing distance signal X_(S) * isnot used for the determination of the estimated velocity V_(S), so thereis no adverse effect from the dead time of the track-traversingdetection circuit 2A or the high resonance-frequency characteristics ofthe head actuator 116. However, even a slightest offset in the drivecurrent I_(L) will increase the estimation error of the estimatedvelocity signal V_(S).

Since the estimated velocity signal V_(S) does not include the trackfluctuation amount Xd at all, the estimate velocity is comprised of anestimated value of the velocity of movement of head actuator 116, ratherthan the track traverse velocity. When the head velocity is low, and thehead velocity V_(L) becomes so low that the track fluctuation velocityVd cannot be ignored, an error which is caused in the estimated velocitysignal V_(S) becomes substantial.

Thus, by setting the parameter L to be sufficiently lower than the highfrequency-band resonance frequency of the head actuator 116 and thetrack traverse frequency, and sufficiently higher than track fluctuationfundamental frequency, the estimated velocity signal V_(S) can be madeto a assume a value which is midway between the value given by formula(26) ad the value given by the formula (29), and the dead time due tothe track-traversing distance detection circuit 2A can be compensated toa certain extent.

On the basis of formula (22), the transfer characteristics from thedifferential value SX_(S) * of the track-traversing distance signalX_(S) * to the estimated-velocity signal V_(S) can be represented by thecharacteristic of the first-order low-pass filter, i.e., 1/(1+S/L), theestimated velocity signal V_(S) will not be disturbed substantially evenif SX_(S) * corresponding to the detected-velocity signal is disturbed,e.g., under the effect of recording bit or drop-outs on the optical disk101.

FIG. 18 is another modification which is an equivalent conversion formthat shown in FIG. 17. Because in both cases the transfercharacteristics of the state-observer unit 3 are equivalent, thedescription of operation of the modifies system is omitted.

FIG. 19 shows a block-diagram of the velocity-control system whichincorporates the state-observer unit 3 of another modification. In thismodification, the gain elements 9, 10 and 21, and the integratorelements 24 and 25 simulate the transfer characteristics of thetrack-traversing distance detection circuit 2A and the head actuator116, which is the object of control. In FIG. 19, the drive current I_(L)of the head actuator 116 is converted by the gain elements 9 and 10 intoan acceleration, and then into a velocity signal by the integratorelement 24, and then into a distance (over which the head has been movedor displaced) by the integrator element 25 and the gain element 21. Thedifference between the displacement-distance signal and output signalX_(S) * of the track-traversing distance detection circuit 2A isdetermined by a subtractor 18A, and is then sent through respectivefeedback gain elements 12 and 23 to adders 14A and 17A where they areadded to the outputs of the gain element 10 and the integrator element24. The outputs of the adders 14A and 17A are inputs to the integratorelements 24 and 25. In this way, the desired velocity control isachieved. In other words, the acceleration and velocity are correctedsuch that the estimated value of the track-traversing distance convergestoward the track-traversing distance detection signal X_(S) *, and thesignal that is obtained by integrating the corrected acceleration signalin the integrator element 24 is the estimated velocity V_(S). For thesystem of FIG. 19, the transfer characteristic from the track-traversingdistance detection signal X_(S) * and the drive current I_(L) of thehead actuator 116 to the estimated velocity V_(S) can be represented bythe following formula: ##EQU11##

Similar to the cases of FIGS. 17 and 18, if the boundaries between thefrequency-bands is denoted by √L₂ of the state-observer unit 3 will beapproximately as follows:

(I) When ω<<√L₂, V_(S) ≃SX_(S) and

(II) When ##EQU12##

Thus, in the case of FIG. 19, the effects will be the same as in thesystems of FIGS. 17 and 18.

Because the state-observer unit 3 of FIG. 19 has in general the sameorder (i.e., the same number of integrator elements) as the headactuator 116, it may be called a "same-order state-observer unit". Ascompared to this, the state-observer unit 3 of FIGS. 17 and 18 have theorder one less than the head actuator 116, and therefore they can becalled "minimum-order state-observer units". It is also known that bymeans of the so-called Gopinath method, the same-order state-observerunit of FIG. 19 can be converted into a minimum-order state-observerunit of the state-observer unit of the type shown in FIG. 17 and 18.

In addition, during tracking of the tracks by the light spot 115, theintegrator elements 13, 22, 24, and 25 (which are shown in FIGS. 17 and19) are reset by a command from the control-mode detection circuit 4.This resetting is cleared simultaneously with the switching into thevelocity-control mode. Since the track-traverse velocity during thetracking is certainly equal to zero, there will be no error in theinitial value of the estimated velocity output from the state-observerunit 3 at the time immediately after the transition into thevelocity-control mode. Moreover, even if an error occurs, the error iswill be reduced to zero with the time constant 1/L or 1/√L₂.

The open-loop transfer functions of the velocity control system shown inFIG. 17 can be obtained by calculation in accordance with the followingformula: ##EQU13##

If the high-band resonance characteristic G_(L) (S) of the head actuator116 has a peak at a frequency ω_(L), by setting L to satisfy L<<ω_(L),then the following relation holds: ##EQU14##

The influence which is exerted by the value of the high frequency-bandresonance peak of the head actuator 116 on the open-loop characteristicof formula (33) will be suppressed to L/ω_(L) times (L/ω_(L) <<1), andas shown in FIG. 22, the high frequency-band peaks of the gaincharacteristics will be smaller than those shown in FIG. 6.

The resultant characteristics is also different from the frequencycharacteristic of a notch filter 122 shown in FIG. 4, and the high-bandresonance frequency ω_(L) need not be a specific frequency. Generally,if the frequency satisfies condition ω_(L) <<L, the suppression effectwill be obtained at an arbitrary frequency, and will be uniform even inthe presence of several peaks. Similar to mechanical resonancecharacteristic G_(L) (S), the phase delay and the gain drop due to thezero-order hold characteristic of the track-traversing distancedetection circuit 2A, in formula (33) are alleviated. Therefore, thephase of the open-loop characteristics extends to the high band, and thestability of the system is improved.

The method of detecting the track-traverse direction by the directiondetection circuit 1 is similar to that described with reference to FIG.12. Responsive to the detected track-traversing direction, the polarityof the track-traversing detection signal of the track-traversingdistance detection circuit 2A is switched, and accumulated positively(counted up) or negatively (counted down). Thus, a positive feedback ofthe control system is avoided, and a stable operation is ensured.

In the above-described embodiment, the track-traversing distancedetection circuit switches the polarity of the track-traversing signaland performs the positive or negative accumulation on the basis of thedetected track traverse direction. Where, during velocity control,reversal of the track traverse direction does not occur or occurs onlyfor a short period, and because, as shown by formula (29), the estimatedvelocity is determined substantially by the drive current, I_(L), thepositive feedback does not take place in the velocity-control system,the direction detection circuit may be omitted.

In the embodiments of FIGS. 17, 18 and 19, the head actuator was alinear actuator which moves linearly. The head actuator mayalternatively be a rotary-type actuator. In this case, the mass M of themovable parts should be replaced by inertia moment J of movable parts.Where the optical head is a separate type in which part only of theoptical head is movable, the head actuator may be designed to move themovable part of the optical head. What is essential is that it iscapable of moving the light spot over a large distance in the radiationdirection of the disk. It may be of such a construction that can serveboth as the tracking actuator 112 (in FIG. 17) and the head actuator.

In the embodiments described, a drive current detection means is usedfor detecting the acceleration of the head actuator. But theacceleration may alternatively be detected by an acceleration sensorattached to the movable part of the head, and its output may be suppliedto the state-observer unit.

Furthermore, in the embodiments described above, the track traversespeed and the track traverse direction are detected on the basis of thesignals obtained by determining the sum and the difference on thesplit-photodetector outputs. But they may obtained in other ways. Forexample, when a sample servo system using an optical disk without trackgrooves is employed, the track traverse speed and the track traversedirection may be detected on the basis of an output of the trackingsignal (tracking error signal) detection means or an output of a tracktraverse number detection means and an output of a means for detecting asignal corresponding to the reflected-light total-amount signal.

Furthermore, they may be detected from the address information or thelike of the optical disk.

As has been shown above, the velocity-control system of the optical diskdrive device described above is provided with a state-observer unit. Atthe time of track access, the track-traversing detection signal from thetrack-traversing distance detection circuit is integrated with itspolarity changed under the effect of the output from thedirection-detection circuit. This signal and the drive current signal ofthe head actuator are then input to the state-observer unit, whichestimates the track-traversing velocity, and the actual track-traversingvelocity of the light spot is controlled on the basis of theabove-mentioned estimation. Such a velocity-control system ischaracterized by an improved stability of operation and provides, at thetime of access, reliable velocity control with long distances. It isalso possible to achieve velocity control with extremely shorttraversing distances. The system makes it possible to considerablyreduce the access time.

The above configuration is efficient because the performance of thevelocity-control system do not depend on such factors as manufacturingfluctuations of the mechanical resonance frequencies and the number ofthe mechanical resonance points of the head actuator and the opticalhead. Accordingly, the assembly of the head actuator and the opticalhead is facilitated.

FIG. 23 is a block diagram of an information storage device of anotherembodiment of the present invention.

A disk motor 402 is rotated under the control of a disk motor-controlcircuit 403. The disk motor 402 drives a spindle which carries anoptical disk 401. Located beneath the optical disk 401 is an opticalhead 404 which can be moved in the radial direction of the optical disk401 by means of a head actuator 308. The optical head 404 is providedwith an optical system which is similar to a conventional one andcomprises a source of light 406, an objective lens 411, and asplit-photodetector 413. Similar to the conventional device, an outputfrom the split-photodetector 413 is sent to an input of anaddition/subtraction amplifying circuit, not shown.

The optical head 404 has a frame 405 having light-blocking plates 302and 304 provided on the lower surface of the disk inner edge and thedisk outer edge of the frame 405. Movement of the optical head 404 inthe disk radial direction is limited by engagement of part of theoptical head 404 with inner and outer stoppers 451 and 452.

Head position detectors 301 and 302 are provided to cooperate with thelight-blocking plates 303 and 304 to detect the presence of the opticalhead 415 is at specific positions between the inner and outer limits(where the optical head 404 engages with the stoppers 451 or 452), andthe limits of the user utilizable region of the disk (the region thatcan be utilized by the user).

In the illustrated embodiment, the head position detectors 301 and 302are of the optical type, and the presence of the optical head 404 in arespective position is determined when the path of light is interruptedby the light-blocking plates 303 and 304.

A position control command generation circuit 309 generates commands Aand B input to the position control loop on/off command circuit 305 forinitiating radial position control over the optical head 404. Thecommands A and B are sent to a position control loop on/off commandcircuit 305. The command A is an inner position control command whichinitiates inner position control. The command B is an outer positioncontrol command which initiates outer position control.

Output signals of the head position detectors 301 and 302 are also sentto the input of the on/off command circuit 305. This on/off commandcircuit 305 produces on its output an on/off command of the positioncontrol loop. This output is sent to an input switching circuit 306. Onthe basis of its input, this circuit selects either an output signalfrom the head position detector 301, or an output signal from the headposition detector 302, and then sends the signal to a stabilitycompensation circuit 307, which is provided for stabilization of theposition control system.

Reference numeral 320 designates a tracking control circuit, 330 is aseek control circuit for control over seeking operation of an opticalspot 415 to a desired track. Output signals of these circuits, as wellas the output of the stability compensation circuit 307, are sent to acontrol mode switching circuit 340. The control mode switching circuit340 is responsive to an output signal of the on/off command circuit 305for selecting one of the above-mentioned three input signals, andswitches the control mode between (a) tracking control mode, (b) seekcontrol mode, and (c) position control mode. The selected signal is sentto a head actuator drive circuit 350. The head actuator 308 is driven bya signal obtained from the drive circuit 350, and moves the optical head404 in the radial direction of the optical disk 401.

FIG. 24 is a circuit diagram which shows details of the head positiondetectors 301 and 302, the position control loop on/off command circuit305, and the input switching circuit 306.

The head position detector 301 comprises a photointerrupter(photocoupler) 311 located in the area over which the light-blockingplate 303 is moved. Similarly, the head position detector 302 comprisesa photointerrupter (photocoupler) 312 located in the area over which thelight-blocking plate 304 is moved. Light-emitting diodes 311b and 312bof both photointerrupters 311 and 312 are connected in series, so thatthey can be driven from a constant-voltage current source 313 of thehead position detector 302.

The collector of a phototransistor 311a, which is included in the innerposition detector 301, is connected to a power supply node 390, whileits emitter is connected to a buffer amplifier 314. The output of thebuffer amplifier 314 is sent to the input of an inner potential controlloop on/off control circuit 316, which is included in the on/off commandcircuit 305, as well as to a contact 305b in the input switching circuit306.

The emitter of a phototransistor 312a, which is included in the outerhead position detector 302, is connected to the ground potential node,while its collector is connected to the input of a buffer amplifier 315,and via a resistor to the power-supply node 390. The output of amplifier315 is connected to the input of an outer position control loop on/offcontrol circuit 317, which is incorporated in the on/off command circuit305, as well as to a contact 306a of input switching circuit 306. Theoperation of the contacts 306a and 306b are opposite to each other. Thatis--when the contact 306a is ON, the contact 306b is OFF; and when thecontact 306a is OFF, the contact 306b is ON.

FIG. 25 is a schematic diagram of the inner position control loop on/offcontrol circuit 316.

A level detection circuit 321, which receives on its input an outputsignal of buffer amplifier 314, may comprise, e.g., a so-called windowcomparator, which decides whether the level of the input signal is in aspecified range. If it is in the specified range, it outputs an innerposition detection pulse 324, which is sent to an OR gate 322. Anotherinput on the OR gate 322 is the inner position control command A. Anoutput signal of the OR gate 322 is applied to a trigger terminal T of aD-flip-flop 323. A terminal D of D-flip-flop 323 is fixed on a "High"level, while a reset terminal R receives a reset command from positioncontrol command generation circuit 309. Thus, a "Set" output Q ofD-flip-flop 323 forms an inner position control loop on/off controlcommand and serves as a control signal for switching for the contact306b.

The outer position control loop on/off control circuit 317 has the sameconstruction as the inner position control loop on/off control circuit316 described above.

Operation of the device of the above embodiment will now be described.

Assume that, while the light spot is moved or positioned by means of theseek control circuit 330 or the tracking control circuit 320 for drivingthe head actuator 308 or the tracking actuator 412, the light spot movesout of the user utilizable region. If, for example, the light spot movesout toward the inner periphery, and the light path of photointerrupter311 of the inner head position detector 301 is interrupted by theblocking plate 303, then a position control loop-on command is issuedfrom the position control loop on/off command circuit 305, and theoutput of the inner position control circuit 1 is selected in the inputswitching circuit 306. This output is then transmitted to the controlmode switching circuit 340 via a stabilization compensation circuit 307formed for example of a phase-lead compensation circuit, etc.

In the control-mode switching circuit 340, when the position controlloop in the position control loop on/off command circuit 305 is turnedON, the seek control, or tracking control, whichever has been conducted,will be turned OFF, and the output signal of the stability compensationcircuit 307 will be transmitted to the head actuator drive circuit 350,and the head actuator 308 will be driven in such a manner that the lightspot is moved to and held at a position where the output voltage ofinner head position detector 301 coincides with the position of apredetermined reference voltage.

A similar operation is performed when the light spot moved out towardthe outer periphery is similar.

The above-outlined operation will next be described in further detail.

Because output of the photointerrupter 311 shown in FIG. 24 is derivedfrom the emitter, and the output of the photinterrupter 312 shown inFIG. 24 is derived from the the collector, the relationship between theamount of passing light and the output of the photointerrupter 311, andthe relationship between the amount of passing light and the output ofthe photointerrupter 312 are reverse to each other. When the light spot415 of the optical head is within the user utilizable region, the outputsignal of the buffer amplifier 314 will be raised to the "High" level,while the output signal of the buffer amplifier 315 will remain on the"Low" level, as shown in FIG. 26.

In FIG. 26, an abscissa shows the position of the optical head 404, andhence, the light-blocking plate 303 or 304. When the light-blockingplate 303 is shifted toward the inner periphery (toward the left in FIG.26), then as shown in FIG. 26(A), the output signal of the bufferamplifier 314 is gradually decreasing, and will be at the "Low" levelwhen the light is completely blocked.

When the light-blocking plate 304 moves toward the outer periphery, andinterrupts the light path in the photointerrupter 312, the output signalof the buffer amplifier 315 is gradually increasing, and when the lightis blocked completely, the output signal of the buffer amplifier 315 isat the "High" level.

These signals are received at the input of the level detector 321 in theinner position control loop on/off control circuit 316. In the leveldetector 321 is set an appropriate threshold value between the "High"and "Low" levels of the buffer amplifier 314. The detector thereforeproduces an inner position detection pulse 324 (FIG. 26(B)). In asimilar manner, an outer position detection pulse (FIG. 26(D)) is formedin the outer position control loop on/off control circuit 317.

FIG. 27 shows the time variation pattern of the output signal of thebuffer amplifier 314 at (A), the inner position detection pulse 324 at(B), and the "Set" output signal Q of D-flip-flop 323 at (C). When theoptical head 404 moves toward the inner periphery of the disk, andlight-blocking plate 303 blocks the photointerrupter 311, the outputsignal of the buffer amplifier 314 (FIG. 27(A)) decreases, which raisesthe inner position detection pulse 324 (FIG. 5(B)). As a result,D-flip-flop 323 is "Set", and the output Q is raised to the "High" level(FIG. 27(C)).

This "Set" output signal Q is sent to the contact 306b of the inputswitching circuit 306, and the output signal of buffer amplifier 314 istransmitted to the input of stability compensation circuit 307. When theoptical head 404 moves further inside due to inertia, the output ofbuffer amplifier 314 goes down to the lower limit of the level detector321, and the inner position detection pulse 324 falls to the lowerlevel, but the D-flip-flop 323 will still be maintained at the "Set"state.

When the output signal of the buffer amplifier 314 is sent to stabilitycompensation circuit 307, the head actuator 308 is automatically drivenso that the level of this signal coincides with a target voltagerepresenting the target position along the radial direction that is setin the control circuit system (FIG. 26(A)).

The optical head 304 whose light-blocking plate 303 has passed the innerhead position detector 301 and is at the inside of the inner headposition detector 301, the optical head is then returned to the positionof the inner head position detector 301, and is held there.

In this way, even if the light-blocking plate 303 passes the headposition detector 301 while the optical head 404 is moving toward theinner periphery of the disk, the optical head 404 returns to the headposition detector 301. It is thus protected from collision with thestopper 451.

Apart from preventing the collision of the head with the stopper 451under the "run-away" condition, the device of the present invention alsostabilizes pull-in action into the operation of the focusingservo-system directly after power-on. This feature will now be describedin more detail.

FIG. 28(A) shows an output signal of the buffer amplifier 314, FIG.28(B) shows an inner position control command A, and FIG. 28(C) showsthe "Set" output signal Q of the D-flip-flop 323. When the optical head404 is at the innermost position in the optical head, at the time ofpower-on, the light-blocking plate 303 completely blocks the light pathin the photointerrupter 311 of the inner detector 301. As a result, theoutput signal of the buffer amplifier 314 is at the "Low" level.

At the time of power-on, for the purpose of reading information from thecontrol track, the position control command generating circuit 309produces the inner position control command A (which is shown to be inthe form of a pulse), which is supplied to the inner position controlloop on/off circuit 316. This, in turn, triggers and sets theD-flip-flop 323, and the output signal Q is set to the "High" level. Inthe same manner as has been described earlier, the head actuator 308will be driven such that the output signal of the buffer amplifier 314will coincide with the target voltage shown by broken line in FIG.27(A), and the optical head 404 will assume a position corresponding tothat of the inner head position detector 301.

If the head position detector 301 is so positioned that the light spot415 is then on the control track on the inner periphery side of the disk404, even if the apparatus is installed inclined or some external forcesare exerted at the time of power-on, it is ensured that the light spotbe on the control track, and the information in this track can be readwithout fail. This enables subsequent controls to be performedcorrectly.

Where the optical head 404 is installed at the outer position, a similarcontrol can be performed. For this purpose, the position control commandgenerating circuit 309 may be arranged to produce an outer positioncontrol command B at the time of the power-on.

FIG. 29 shows a positional relationship between the light-blocking plate303, the photointerrupter 311 and the output signal buffer amplifier314. Where the inner edge of the light-blocking plate 303 isperpendicular to the direction of movement, within the boundary of thephotointerrupter's width a the output signal will be changed linearly.

As compared to this and as shown in FIG. 30, with the inner edge oflight-blocking plate 303 inclined at a certain angle θ to the directionof movement, the output signal of the buffer amplifier 314 will varylinearly within the range of (1+1/tan θ)a.

Finally, an expansion of the linear zone decreases the amount ofovershooting in the position control circuit, and thus will shorten thesetting time.

A solid line in FIG. 31 shows the condition under which photointerrupter311 is not blocked by light-blocking plate 303, while the phantom linein this drawing shows the condition under which photointerrupter isblocked half way. The area of photointerrupter 311 which receives theincident light in the unblocked position is a square with the side equalto a, while in the latter case, the light is shaded on the area which inthe vertical direction is defined by length y on the inner side, and bylength x on the outer side. The following relationships can be writtenon the bases of this designation:

    x+y=a                                                      (41)

    x=y+a tan θ                                          (42)

    x=l/2 tan θ                                          (43)

Where l is the length of movement of light-blocking plate 303 betweenthe solid line position and the position in which the light iscompletely blocked.

The following expression can be obtained from equations (41) to (43):##EQU15##

Formula (44) is an expression which shows an expansion factor of thelinear area for head position detector 301. In the case of θ equal to45°, the expansion factor is equal to 2, as compared to the case withθ=90°.

Such an expansion is identical both for the direction toward the outerside of light-blocking plate 304 and toward the outer side of headposition detector 302.

The above-mentioned embodiment was considered for the case where theposition of the optical head is detected by using a light-passingphotointerrupter and a light-blocking plate. However, the same resultscan be obtained with the use of a reflection-type photoreflector and areflecting plate.

In the case of a reflection-type element, the signal will have polarityopposite to that obtained in the case of a light-passing element. Theinvention is not limited to application of only a photointerrupter, or aphotoreflector, and can be realized with the use of light-emitting andlight-receiving elements. In addition, in the case of the reflectionsystem, the reflecting plate can be substituted by an appropriate mark(inside of which the reflection is smaller than outside of the mark)applied onto optical head 404. Enlargement of the zone with linearcharacteristics of the output signal is equally applicable for the caseof a reflecting plate, as well as a mark.

Light-blocking plates on the inner and outer sides can be combined withsetting positions of head position detectors 1 and 302. Moreover, headposition detectors 301 and 302 can be installed on the optical head,while the light-blocking plates 303 and 304 will be fixed.

Instead of an optical principle, the head position detectors mayoperates on a magnetic principle, or may have any other suitableconstruction.

In the illustrated embodiment, stability compensation circuit 307 isbased on a phase-lead compensation circuit, but the stability can becompensated also through the velocity feedback line.

D-flip-flop 323 of the position control loop on/off command circuit canbe substituted by an R-S flip-flop, or by another latch circuit.

The principle of the invention can also be realized on the basis of acombined use of tracking actuator 412 and head actuator 308. Only a partof the optical head can be made movable.

The head actuator may be a linear-type or a rotary-type actuator.

The invention above embodiment of the is not limited only to opticaldisk devices. The same principle is applicable to information storagemedia on magnetic disks, or similar devices which can be traversed bythe magnetic head.

It has been shown that when the head is moved away from the workingposition, it will be moved back to the position determined by theposition detector, so that it will be protected from the collision withthe stopper, and hence from breakage.

FIG. 32 illustrates a system in accordance with another embodiment ofthe invention. In FIG. 32, reference numerals 201 to 207 designate thesame elements as in the previously described known system shown in FIGS.7 and 8. Reference numeral 230 designates tracking control means, whichon the basis of output signals from the photodetector 207, guide thelight spot, which is formed by the beam 202, along the center of thetrack on the optical disk 201. A track traverse velocity detection means240 detect the track traverse velocity (the velocity with which thelight spot traverses the tracks) on the basis of the output from thephotodetector. A reference velocity generation means 250 generate areference velocity which forms a target value for the track traversevelocity (the velocity with which the light spot traverses the tracks).A speed control means 260 control the drive of a linear actuator 205, sothat the track traverse velocity detected by the track traverse velocitydetection means 240 coincides with the reference velocity, which isoutput from the reference velocity generation means 250. An off-trackdetection means 270 detects off-track (departure of the light spot fromcorrect or target track) during movement of the light spot along thecenter of the track and produces an off-track signal upon detection ofthe off-track. A mode switching command generation means 280 makeswitching between the tracking control mode and the velocity controlmode on the basis of the track traverse velocity detected by tracktraverse velocity detection means 240 and the off-track detection signalobtained from the output of the off-track detection means 270. A controlmode switching means 290 switches the control mode of the trackingactuator 206, and the linear actuator 205, depending on the command fromthe output of the mode switching command generation means 280.

FIG. 33 is a block diagram of a velocity control system which controlsthe track traverse velocity in the device of the above-describedembodiment. In FIG. 33, reference numerals 205, 240, 250 and 260designate the same elements as those shown in FIG. 32, reference numeral261 designates a stability compensation unit which is intended forstabilization of the velocity control system and located inside velocitycontrol means 260. In general, a gain element is used for this unit. Adrive circuit 262 is intended for driving the linear actuator 205. Ingeneral, a constant-current drive circuit is used for this circuit.Operating current i_(LM) is is caused by the drive circuit 262 to flowthrough the linear actuator 205, so that the linear actuator 205 movesat the velocity V_(LM). The sum of the objective lens velocity V_(LENS)and the linear actuator velocity V_(LM) makes the absolute velocityV_(SPOT) of the light spot, and the difference between the absolutevelocity V_(SPOT) and the track fluctuation velocity V_(DISK) of theoptical disk 201 makes the track traverse velocity V_(CROSS). The tracktraverse velocity detecting means 240 detects the track traversevelocity V_(CROSS). The signal indicative of this detected tracktraverse velocity is denoted by V_(CROSS). Because it is difficult toaccurately detect the correct track traverse velocity on the basis ofthe output from the photodetector 207 alone, the drive current i_(LM) ofthe linear actuator 205 may be used in combination.

FIG. 34 is a diagram which is used for a detailed explanation of theoff-track detection means 270 and the mode switching command generationmeans 280, used in the device of the above-described embodiment.

In FIG. 34, reference numerals 207, 240, 270 and 280 designate the sameelements which are shown by these reference numerals in FIG. 32. Thetracking-error detecting means 221 receive the output of thephotodetector 207 and produces a tracking-error signal. The totalquantity detecting means 222 receives the output of the photodetector207, detects the total quantity of the reflected light, and produces atotal reflected light quantity signal. The comparator 271 compares thelevel of the tracking-error signal obtained from the tracking-errordetection means 221 with V₀. The comparator 272 compares the level ofthe tracking-error signal from the tracking-error detection means 221with -V₀. The comparators 271 and 272 detect deviation of the light spotfrom the center of the track exceeding a predetermined distance, andproduces on its output a logic signal indicative of the deviationexceeding the predetermined distance. Inverters 273 and 274 invert thepolarities of the output signals of the comparators 271 and 272. An RSflip-flop 275 receives on its set and reset terminals respective outputsignals of the inverters 273 and 274. A timer 276 issues a gate signalof a predetermined duration T (FIG. 36(J)), the gate signal being raisedupon transition of the output of the RS flip-flop 275 from the low tohigh level. A counter 277 is loaded with a predetermined value, e.g.,"0" and counts down by "1" each time a pulse is applied to its C/D inputfrom the RS flip-flop 275 (each time its input to its C/D terminal fallsfrom High to Low), and when the result of the count down becomesnegative, it produces on its output (B) a Borrow signal. (Because thecounter produces the Borrow signal when its count value becomes negative(-1) and this Borrow signal is utilized for the generation of theoff-track signal, the above-mentioned "predetermined value" should beone short of the number of the tracks upon traverse of which theoff-track signal is desired to be produced.) This Borrow signal isapplied to an AND gate 278 and is ANDed with the output of the timer276. The output (E) of the AND gate 278 is the off-track signal which isproduced each time the light spot that has deviated from the targettrack traverses other (neighboring) tracks. This Borrow signal isapplied through an AND gate 284 being opened because of its other input(K) being High to a set terminal of an RS flip-flop 286, to set theflip-flop 286, and its output (H) is raised to High.

The off-track signal (E) is also applied to the load terminal (LD) ofthe counter 277 to load the counter with the predetermined value, e.g.,"1." The counter 277 then starts counts down from the predeterminedvalue again.

The function of the timer 276 is to disregard any single high to lowtransition (at the output of the flip-flop 275) due for example to adefect on the medium.

A track center detection circuit 281 detects the position of the trackcenter on the basis of output signals from the tracking-error detectionmeans 221 and the total-reflected-light-quantity detection means 222. Avelocity-comparing means 282 detects the track traverse velocity whichis detected by the track-traverse velocity detection means 240, becomeslower than a pull-in velocity (the velocity below which pull-in by thetrack control means 230 is possible). A NAND gate 283 produces a pulsecorresponding to the track center only when the track traverse velocityis below the pull-in velocity (the velocity below which the light spotis capable of being pulled into the tracking control mode (in which itfollows the center of the track under the control of the trackingcontrol means). A seek control logic circuit 291 generates a command forswitching into the tracking control mode, promptly when the light spotreaches the target track during seeking of a track by means of velocitycontrol of the linear actuator 205. This command is generated in theseek control logic circuit 291 responsive to a pulse indicative of thecenter of a track after the counter produces the Borrow signal, and issupplied from the mode switching command generation means 280 to thecontrol mode switching means, 290. An OR gate 284 sets the flip-flop 286(by applying a signal to the set terminal of the flip-flop 286) at thetime when seek-initiation command is received from the seek controllogic circuit 291, or an off-track signal E is received from the counter277. Another OR gate 285 resets the flip-flop 286 (by applying a signalto the reset terminal of the flip-flop 286), when the seek control logiccircuit 291 produces a tracking initiation command, or when a NAND gate283 produces a track center pulse (at the low velocity period). On thebasis of outputs from the OR gates 284 and 285, the RS flip-flop 286issues a command for switching between the tracking control mode and thevelocity control mode.

FIG. 35 illustrates details of the track traverse velocity detectionmeans 240. In FIG. 35, reference numerals 207, 221, 222, 240 and 262designate identical parts shown in the previous drawings. A tracktraverse speed detection means 241 detects the track traverse speed (A)from the tracking-error signal. A direction detection means 242 detectsfrom the tracking-error signal and the total-reflected-light-quantitysignal, the track-traverse direction (direction in which the light spottraverses the tracks) A polarity switching circuit 243 switches(selects) the polarity of the track traverse speed (A) on the basis ofthe direction detected by direction detection means. A state-observerunit 244 receives on its input an operating (drive) current signalobtained from the output of the drive circuit 262, as well as a tracktraverse detection velocity (a₂) obtained from the polarity switchingcircuit 243. On the basis of the signals received, the state-observerunit produces an estimated track traverse velocity (a₁) which is moreaccurate (or has a smaller delay) than the detected velocity (a₂).

Operation of the system made in accordance with the above embodimentwill now be explained with reference to FIG. 36 to FIG. 38. In thesedrawings, FIG. 36 is used for explaining the case of an off-track underthe effect of impact or a similar external factor which may affect thedevice during tracking. FIG. 37 is a drawing which explains an off-trackdue to a failure in track-jump operation. FIG. 38 explains operation inthe case of an off-track because of a failure in the operation ofpull-in by the tracking servo system immediately after a macroseekingoperation.

In FIGS. 36 to 38, (A) designates a track traverse speed, (B) designatesa tracking-error signal, and (C) and (D) designate results of comparisonmade by the respective comparators 271 and 272, specifically the outputsof the inverters 273 and 274, shown in FIG. 34. (E) designates anoff-track detection signal, (F) is an output of the velocity comparingmeans 282 which show that the track traverse velocity is lower than thepull-in velocity. (G) designates an output of the NAND gate 283, (H)designates control mode switching command, (I) designates a track-jumppulse, and (J) designates an actual track traverse velocity.

The system described above operates in the following manner: First, asshown in FIG. 32, any off-track, which may occur in the course ofmovement of the light spot along the center of a track during thetracking control mode, is detected by the off-track detection means 270on the basis of an output from photodetector 207. The off-trackdetection signal is transmitted to the mode switching command generationmeans 280. As a result, the control mode switching means 290 is switchedto assume the state opposite to that shown by the arrows (in FIG. 32),so that the system is switched from the tracking control mode ofoperation performed mainly by the tracking actuator 206, to the velocitycontrol mode of operation by the linear actuator 205.

In this velocity control mode of operation, the reference velocitygeneration means 250 generate a reference velocity which is lower thanthe pull-in velocity (the velocity below which which the light spot canbe pulled into the tracking mode under the control of the trackingcontrol means 230). For example, when the reference velocity generationmeans produces an output equal to zero, the track traverse velocity iscontrolled by the velocity control means 260 so that the track traversevelocity as detected by the track traverse velocity detection means 240coincide with the reference velocity. In other words, the track traversevelocity is automatically decreased to zero. At the same time, the modeswitching command generation means 280 keeps monitoring the output ofthe track traverse velocity detection means 240, and when it is detectedthat the light spot has reached the center of a track immediately afterthe track traverse velocity has fallen below the pull-in velocity, onthe basis of the output of the photodetector 207, the tracking controlmode command is generated at the seek control logic circuit 291 andsupplied through the the mode switching command generation means 280 tothe control mode switching means 290, which is thereby is switched tothe tracking control mode, and tracking control is restarted.

Operation of the system shown in FIG. 34 will now be described in detailwith reference to FIG. 36.

If in the course of the track control operation, for some unexpectedcause, such as an impact, or a crack on the medium surface of theoptical disk 201, the target track may be missed at point b₁ (FIG. 36B),then the track traverse speed will be detected as a reciprocal of thetime required for a half-period of the tracking-error signal (B), andthe track traverse speed will be detected for the first time when thelight spot is at a half-tracking-error point b₂ (at a point midwaybetween the target track and the adjacent track). Meanwhile, thecomparators 271 and 272 compare the level of the tracking-error signal(B) with V₀ and -V₀ and generate on their outputs track reversedetection pulses (C), and (D). With the use of the RS flip-flop 275,each track traversed by the light spot will be counted down by thecounter 277 which will produce a pulse on its output. At the same time,the timer 276 will be reset. For example, if the counter 277 waspreliminarily loaded with "1," each time one track is traversed, anoff-track detection signal (E) is produced. Under the effect of thissignal, the counter 277 will be again loaded with "1," and the RSflip-flop 286 will be set. As a result, the control mode switchingcommand signal (II) will become a velocity control command. In thevelocity control mode, the velocity control means will reduce the tracktraverse velocity until it coincides with the reference velocity (equal,e.g., to zero), and in the center of the track (point b₄) where thelight spot is positioned at the center of the track immediately afterthe time point b₃ where the reduction of the velocity below V_(SH)(below at which the entering into the tracking operation becomespossible), the track center pulse (G) will reset the RS flip-flop 286,the control mode switching command signal (II) will become the trackingcontrol command, and the light spot will be safely pulled into thecenter of a track (b₄) in the vicinity of the track from which theoff-track has occurred. FIG. 36 shows the case in which the light spotis pulled into a center of a track separated by five tracks from theinitial track. If the light spot cannot enter the center of the track inpoint b₄ as well, e.g., because of a crack in the medium, the off-trackwill be detected again, the speed will be reduced through the velocitycontrol mode, and the above-described operation will be repeated untilthe light spot safely enter the tracking operation.

FIG. 37 illustrates the case where an off-track occurs in track jump.When the tracking actuator 206 is driven under the command of atrack-jump operation and an off-track occurs, then similar to the caseof FIG. 36, the off-track will be detected at the moment t₁ whichcorresponds to departure by 1 track from the center of the initialtrack. At the same time, the system will be switched to the velocitycontrol mode, and by means of driving the linear actuator 205 the tracktraverse velocity is reduced. At point t2, when the velocity becomesbelow the pull-in velocity, the tracking control means 230 will thenagain switch the system to the tracking control mode, and in point b astable tracking becomes possible. This point b is four tracks away fromthe target track to be reached by the track jump operation. In order topass from this track position b to the target track, a seek operationover four tracks in the reverse direction is made, taking the timeperiod of from point t₃ 3 to point t₄, under the command of the seekcontrol logic circuit 291. In this way, the target track is quicklyreached.

FIG. 38 illustrates the case where immediately after a macroseekingoperation with drive of the linear actuator 205, the system could notenter the tracking. Because at time t₁, the light spot has reached thecenter of the target track during a macroseeking operation, the controlmode switching command (II) will be switched from the velocity controlmode to the tracking control mode. Because at this moment, however, anactual tracking traverse velocity (J) is higher than required, the lightspot cannot be pulled into the center of the target track, and this willcause an off-track. The off-track is detected at time point t₂, and thesystem is switched to the velocity control mode, at time point t₃, whenthe velocity is sufficiently reduced, the system will again be switchedto the tracking control mode, so that the light spot will safely enterthe tracking position. After that, the macroseeking operation, or thetrack-jumping operation is repetitively made in the reverse direction,for the same number of tracks for which the light spot has surpassed,similar to the case of FIG. 37.

An example of the velocity detection operation under the velocitycontrol mode will now be described with reference to FIG. 35.

On the basis of an output signal from the photodetector 207, thetracking-error detection means 221 generates a tracking-error signal.The track traverse speed detection means 241 measures the half-period ofthe tracking-error signal, calculates the reciprocal of this value, andthus determines the track traverse speed. However, this track traversespeed is an absolute value of the velocity. The direction detectionmeans 242 on the other hand detects the track traverse direction. Theresults of this detection are used for deciding the polarity of thetrack traverse speed signal (A) and applied to the polarity selectingcircuit 243. The polarity selecting circuit 243 combines the speed withthe polarity. In other words, it vectorizes the track traverse speedinto a track traverse velocity signal a₂. The velocity may be controlledby simply making the signal a₂ to coincide with the reference velocity.However, the velocity control characteristics will be further improvedwith the use of the state-observer unit 244, which will produce anestimated track traverse velocity a₁ on the basis of theoperating-current information of the linear actuator 205 obtained fromthe drive circuit 262, and performs the control such that the velocitya₁ approaches and coincides with the reference velocity. For thevelocity signal used for the judgement of whether or not the tracktraverse velocity is at such a value as to enable the pull-in, eitherthe output of the track traverse velocity detecting means 41 or theoutput of the state-observer unit 244 may be used.

FIG. 39 shows a modified form of the system of the present invention.Because in this drawing, the reference numerals identical to those ofFIG. 32 designate the same elements, their description is omitted. Alaser diode 209 is used as a source of light. An optical system 208includes a mirror for re-directing the light beam from the laser diode209, and an objective lens for focusing the light beam 202. The modifiedsystem differs from the embodiment of FIG. 32 in that the mass ofmovable parts is reduced by separating the photodetector 207 and thelaser diode 209, which function as the light source, from the remainingelements of the optical head, so that only the linear actuator 205participates in the tracking motion. The output signals of the controlmode switching means 290 are transmitted only to the linear actuator205. As a result, there is no need for the linear actuator to drive theentire optical head, but rather only its part.

In the modified embodiment, the head actuator comprises alinearly-movable voice-coil type linear motor, but invention is notlimited only to this embodiment. For instance, the head actuator maycomprise a swing-arm type rotary actuator. Although a tracking actuator206 is used for reduction of the track traverse velocity, it can besubstituted by a linear actuator 205 with a low range of motion, so thatthe track traverse velocity can be controlled in a wider scope.

In the embodiment illustrated above, the system was described withreference to an optical disk. It is understood however that theinvention is not limited only to optical disks and that is equallyapplicable to magnetic disks or optical cards, provided that they areused in information storage devices and allow for recording, reproducingand erasing on or from the arbitrary information record medium havingmultiple tracks. The circuitry of the system can be reduced, ifimmediately after tracking deviation control the same velocity controlmeans are used for the seeking operation.

As has been described above, in the information storage device describedabove with reference to FIGS. 32 to 39, when an off-track occurs duringtracking control this is detected and the control mode is switched intothe velocity control mode, and after the track track traverse velocityhas been regulated, the control mode is again returned to the trackingcontrol mode. Accordingly, when a large impact is applied from outsideor when an off-track occurs due to a large defect or scratch on theinformation storage medium, or when pull-in falls immediately afterseeking, run-out of the head is prevented, and pull-in into a track inthe vicinity of the target track can be achieved. It should be notedthat this is achieved without resorting to an external scale as in theprior art. Thus, an information storage device which operates at a highspeed and which is more reliable is obtained.

What is claimed is:
 1. An information storage device capable ofoperation in a velocity control mode and a tracking control mode andcomprising:a head having a carriage and a tip, for recording,reproducing and erasing information on and from an information storagemedium having multiple tracks; a tracking actuator for moving said tipof the head in a track-traverse direction; a head actuator for drivingsaid carriage in said track-traverse direction so that said tip of thehead traverses the tracks; track control means for controlling, in saidtracking control mode, the tracking actuator so that said tip of thehead follows the center of a track; off-track detecting means fordetecting the departure of said tip of said head from the track; meansfor controlling, in said velocity control mode, the head actuator suchthat the velocity with which said tip of said head traverses the tracksis reduced to a relatively small value; and mode switching commandgeneration means for switching control of said tip of said head betweensaid velocity control mode and said tracking control mode; wherein whensaid off-track detection means detects said departure during trackingcontrol mode, said mode switching command generation means switches tosaid velocity control mode, and when the track traverse velocity isreduced to the small value said mode switching command generation meansswitches to said tracking control mode.
 2. The device of claim 1,wherein said relatively small value is substantially zero.
 3. The deviceof claim 1, wherein during control of the velocity with which the tracksare traversed by the tip of said head under the effect of said headactuator, said velocity with which said tip of said head traverses thetrack is lowered sufficiently to permit tracking to resume on a targettrack.
 4. The device of claim 1, wherein after control of the tracktraverse velocity under the effect of said head actuator the tracktraverse velocity is detected for checking that it is below saidvelocity with which said tracking control mode is entered, and thedevice is again switched to the tracking control mode.
 5. The device ofclaim 1, wherein said information storage medium is an optical disk, andthe tip of said head is a light spot.